1. Field of the Invention
The present invention relates to an optical wavelength tuning method and a Fabry-Perot type optical tuner, which are used in the optical wavelength analyser and the optical wavelength division multiplexing communication.
2. Description of the Related Arts
In recent years, optical fibers, which are able to transmit large amount of data at high speed, have become to be used in place of copper wires.
For instance, as shown in FIG. 9, the multiplexer 101 at the sending side, and the demultiplexer 102 at the receiving side are connected with an optical fiber 103, and the optical signals P1, P2, . . . Pn of respective wavelengths xcex1, xcex2, . . . xcexn, which are converted from electrical signals S1, S2, . . . Sn respectively with electro-optical transducers 104, are multiplexed and transmitted through an optical fiber 103 to a demultiplexer 102 at the receiving side and these optical signals of P1, P2, . . . Pn of wavelength xcex1, xcex2, . . . xcexn are respectively converted to the electrical signals S1, S2, . . . Sn with photo-electric transducers 105. In general, optical signals of the wavelengths 1.3-1.5 xcexcm are used because of their small loss in an optical fiber.
Within this wavelength range, multiple optical signals P1, P2, . . . Pn with slightly different wavelength, are transmitted through an optical fiber 103 and respective optical signals are selectively chosen at the receiving side. This is called Wavelength Division Multiplexing technology.
At present, semiconductor lasers are used to produce optical signals, P1, P2, . . . Pn of wavelengths xcex1, xcex2, . . . xcexn respectively, and to select required optical signal, many optical filters corresponding to respective wavelengths xcex1, xcex2, . . . xcexn are switched by use of an electric motor, and thus an optical wavelength tuner is quite large in size (for instance, a product by OCLI, USA has a size of 10 cm*5 cm*2 cm, and a product by Santec, Japan, 4 cm*2 cm*0.8 cm) and it takes a time in operation and it is quite expensive.
To realize a small and inexpensive Fabry-Perot optical tuner, researches are now going on which use liquid crystal materials. However, these are not practical because of their very narrow tunable ranges, about 30 nm. (cf. xe2x80x9cOptical WDM Networks Principles and Practicexe2x80x9d; editors, Krishna M. Sivalingam and Suresh Subramaniam; Kluwer Academic Publishers, 2000, p.41, Table 2.2)
As mentioned above, conventional technology uses many optical filters and an electric motor is used to select and switch the desired optical wavelength, and thus it is inevitable that a whole device becomes bulky and expensive.
The purpose of the present invention is to provide small and inexpensive optical wavelength tuning method and a Fabry-Perot type optical tuner to obtain an optical signal of the desired wavelength, by controlling the distance between the reflectors of a Fabry-Perot type cavity with the electric voltage applied to the electrodes, and at the same time, by controlling the refractive index of a liquid crystal material filled in between the opposing reflectors.
Optical wavelength tuning method and a Fabry-Perot type optical tuner of the present invention have a special feature in the concurrent use of both the change of the distance between the reflector plates of the Fabry-Perot cavity and the change of the refractive index of liquid crystal material filled in between the reflector plates by applying an electric voltage across the opposing reflector plates of the Fabry-Perot cavity. By this means, an applying voltage needed is order of only a few volts and the synergistic effect of the tuning function due to the change of distance between the reflector plates of the Fabry-Perot cavity and the tuning function due to the change of a refractive index of the liquid crystal material in the cavity gives an excellent tuning effect.
Namely, the first invention is an optical wavelength tuning method in which a tuning to a particular wavelength among the multiplexed optical signals transmitted through a multiplex transmission line to obtain a specific optical signal output is made possible by changing the distance between the reflector plates of a Fabry-Perot type cavity with distance changing unit, and at the same time by controlling the refractive index of the liquid crystal material, which refractive index is a function of the applied electric field thereto, filled in the Fabry-Perot optical cavity by controlling the applied electric field with electric field application unit.
The second invention is a Fabry-Perot type optical tuner comprising, a Fabry-Perot type cavity with variable distance opposing reflector plates, a distance control unit for controlling the distance between the opposing reflector plates, liquid crystal material filled in between the opposing reflector plates and having refractive index dependent on the applied electric field, and an electric field applying unit for applying an variable electric field strength to the liquid crystal material.
The third invention is a Fabry-Perot optical tuner described above, wherein the distance control unit are male screws to fit in the female screws at a transparent substrate plate of the two transparent plates supporting the respective reflector plates, and the distance between the two transparent substrate plates is controlled by turning the male screws.
The fourth invention is a Fabry-Perot optical tuner described above, wherein the distance control unit are the piezo-electric elements connecting the opposing transparent substrate plates to support the reflector plates, and the distance between the transparent substrate plates is changed by applying an electric voltage to the piezo-electric elements to elongate or to shrink the piezo-electric elements.
The fifth invention is a Fabry-Perot optical tuner described above, wherein the distance control unit are the elastic elements connected to the two transparent plates supporting reflector plates, and electrostatic attracting plates fixed at the respective transparent substrate plates, which control the distance between the transparent substrate plates according to the applied voltage against the elastic force of the elastic elements.
The sixth invention is a Fabry-Perot optical tuner wherein the electrostatic attracting plates are the electric field application unit to the liquid crystal material.
The seventh invention is a Fabry-Perot optical tuner, wherein the elastic elements are the flexible spacers between the transparent substrate plates.
This specification includes part or all of the contents as disclosed in the specification and/or drawings of Japanese Patent Applications No. 081914/2001 and No. 154087/2000, which are priority documents of the present application.